Liquid crystal display devices and various other types of display devices are currently used in a variety of applications. In a general display device, a single pixel consists of three subpixels respectively representing red, green and blue that are the three primary colors of light, thereby conducting a display operation in colors.
A conventional display device, however, can reproduce colors that fall within only a narrow range (which is usually called a “color reproduction range”), which is a problem. FIG. 30 shows the color reproduction range of a conventional display device that conducts a display operation using the three primary colors. Specifically, FIG. 30 shows an xy chromaticity diagram according to the XYZ color system, in which the triangle, formed by three points corresponding to the three primary colors of red, green and blue, represents the color reproduction range. Also plotted by crosses X in FIG. 30 are the surface colors of various objects existing in Nature, which were disclosed by Pointer (see Non-Patent Document No. 1). As can be seen from FIG. 30, there are some object colors that do not fall within the color reproduction range, and therefore, a display device that conducts a display operation using the three primary colors cannot reproduce some object colors.
Thus, in order to broaden the color reproduction range of display devices, a technique for increasing the number of primary colors used for display purposes to four or more has been proposed recently.
For example, Patent Document No. 1 discloses a liquid crystal display device 800 in which a single pixel P consists of six subpixels R, G, B, Ye, C, and M representing the colors red, green, blue, yellow, cyan, and magenta, respectively, as shown in FIG. 31. The color reproduction range of such a liquid crystal display device 800 is shown in FIG. 32. As shown in FIG. 32, the color reproduction range, represented by a hexagon of which the six vertices correspond to those six primary colors, covers almost all object colors. By increasing the number of primary colors for use in display in this manner, the color reproduction range can be broadened. In this description, a display device which carries out a display operation using four or more primary colors will be referred to herein as a “multi-primary-color display device” and a liquid crystal display device which carries out a display operation using four or more primary colors will be referred to herein as a “multi-primary-color liquid crystal display device”. Meanwhile, a conventional general display device which carries out a display operation using the three primary colors will be referred to herein as a “three-primary-color display device” and a liquid crystal display device which carries out a display operation using the three primary colors will be referred to herein as a “three-primary-color liquid crystal display device”.
A video signal input to a three-primary-color display device generally has an RGB format or a YCrCb format. A video signal in any of these formats has three parameters (i.e., is a so-called “three-dimensional signal”), and therefore, the luminances of the three primary colors (that are red, green and blue) for use to conduct a display operation are determined unequivocally.
For a multi-primary-color display device to perform a display operation, an image signal with a format for a three-primary-color display device needs to be converted into an image signal with more (i.e., four or more) parameters. Such an image signal representing four or more primary colors will be referred to herein as a “multi-primary-color image signal”. Also, an image signal with a format for a three-primary-color display device, i.e., an image signal representing the three primary colors, will be referred to herein as a “three-primary-color image signal”.